I built a house on rocky terrain and noticed that the game does not display the correct indoor graphics. Instead, it shows the rocky ground graphic, and if it's been snowing, it shows the snow-covered terrain graphic. There's an entry in the list of known bugs that says "Inside of building don't display correctly on rocky terrain during snowing" - I'm not sure if this is the same thing, since this bug happens whatever it is snowing or not (i.e. the rocky ground graphic always shown). The house was originally built when it was snowing (and the indoors showed the snowy ground graphic even after building the roof/floor, except on those tiles that had some other terrain type), and when the summer came and snow melted, it had the usual rocky ground graphic - and now when the winter is coming and it snows, it again shows the snowy ground graphic indoors. EDIT: during the summer I deconstructed one of the inside tiles and rebuilt it, trying to find out whether this had any effect on the graphics - it still showed the rocky ground graphic, and now the snowy ground, so it seems to be related to rocky ground in general.